Ethylene-vinyl alcohol copolymers (hereinafter, also merely referred to as “EVOH”) have an extremely small amount of oxygen transmission in comparison with other plastics and exhibit favorable melt moldability, and therefore have been widely used as food packaging materials and other packaging materials. However, when packaging materials produced using, for example, EVOH are subjected to a retort treatment or used under high-temperature and high-humidity conditions for a long period of time, there are cases of whitening or deformation of the packaging materials or a decrease in barrier properties of the packaging materials, and thus further improvement in hot water resistance is being demanded.
As a strategy for improving the hot water resistance, various techniques have been proposed in which EVOH is crosslinked using an activated energy ray such as an electron beam. For example, Patent Literature 1 discloses a method for melting and kneading EVOH with triallyl cyanurate or triallyl isocyanurate used as a crosslinking agent and then crosslinking the EVOH by irradiation with an electron beam.
In addition, Patent Literature 2 discloses a procedure of adding a compound having two or more allyl ether groups to EVOH and crosslinking the EVOH by irradiation with an electron beam.
Further, Patent Literature 3 discloses a method for modifying EVOH with an epoxy compound having a double bond and an epoxy compound not having a double bond, and crosslinking at least a part of the resultant modified EVOH by irradiation with an electron beam.
Furthermore, Patent Literature 4 discloses a procedure of adding an amide compound having a plurality of double bonds to EVOH and crosslinking the EVOH by irradiation with an electron beam.